Manufacture of cellulosic products



United States Patent @fihce V 3,045,184 Patented July 24, 1962 The present invention relates to a method of producing cellulosic pulp. More particularly it is concerned with a method of delignifying wood chips to produce a cellulosic pulp suitable for the production of paper therefrom.

A substantial portion of the wood pulp produced in the United States by chemical methods at present is made by the acid sulfite pulping process. In the acid sulfite process the pulp is usually made by digesting wood chips at a temperature of about l25-140 C., under about 70-90 pounds per square inch of pressure for periods ranging from about 5 to 19 hours. The cooking liquor ordinarily consists of a solution of calcium, magnesium or ammonium bisulfite containing a large excess of sulfur dioxide and having a pH of approximately 1.5. The process is usually used with coniferous woods having a low resin content, such as spruce, balsam and hemlock. The yields of pulp in a sulfite process average about 47 percent of the weight of the wood. The resultant pulp is characterized by reasonably good strength and easy bleaching characteristics. Sulfite pulps are used in the manufacture of papers where the requirement is a combination of good strength and good brightness. Sulfite pulps may also be mixed with other pulps such as groundwood to supply these characteristics to the finished paper.

It is an object of the present invention to provide a method of producing a cellulosic pulp having substantially greater strength than pulp produced by the conventional acid sulfite process and having bleachability comparable to the acid sulfite pulp.

It is an additional object of the present invention to provide such a pulping process which in part may be carried out in conventional acid sulfite pulping process equipment.

Other objects of the present invention will be apparent from the following description of the invention.

It has been found in accordance with the process of the present invention that a cellulose pulp suitable for the manufacture of a strong, bright paper can be prepared by a novel two stage pulping process. In the first stage Wood fragments are cooked in an aqueous solution of sodium bisulfite having a pH of about 4-5 under the pressure generated by the solution to a yield of about 50-60 percent. In the second stage the resultant pulp is treated with an alkali liquor having an active alkali content of about 20-30 grams of alkali expressed as sodium oxide per liter of solution, at a temperature of about 120-195 degrees centigrade for a period of about 2 /2 hours to a few minutes. In a preferred embodiment in which the first stage of the process may conveniently be carried out in conventional sulfite pulping digestion apparatus, the wood is cooked in'a sodium bisulfite cooking liquor having a pH of about 4-5 under a maximum temperature of ISO-155 C., and a pressure of 90-95 pounds per square inch gauge for a time sufl'icient to provide a yield of pulp in this stage of about 50-55 percent. The resultant pulp is then separated from the liquor and treated with an alkali liquor having an active alkali content of about 20-30 grams of alkali expressed as Na O per liter of solution at a temperature of about 130-170 C., for from about 2 hours to 5 minutes.

A simplified flow diagram of the process of this invention is as follows:

Cal

L Wood chips 7 Pressure digest in aqueous sodium bisulfite liquor containing no tree 50; at pH 4-5 and temperature of 145-165 O. to a pulp yield of 50-60% Screen and wash Extract pulp with aqueous alkaline liquor at temperature of 120-195 C. and pressure of 25-190 p.s.i.g. to a yield for the extraction step of -80% Pulp produced in accordance with the process of the present invention has been found to have tearing strength as much as 2 /2 times as great as pulp made by the conventional acid sulfite pulping process. It is believed that the major factor in the attainment of a tearing strength of this magnitude is the employment in the first stage of the present process, of a sodium sulfite pulping liquor which is acid but contains no free sulfur dioxide. Conventional sulfite process liquors have a pH of approximately 1.5 and contain a substantial amount of free sulfur dioxide. The sodium base sulfite cooking liquor of the first stage of the present invention which has a pH of about 4 to 5 contains no free sulfur dioxide. While free sulfur dioxide is necessary in the calcium bisulfite liquors employed in conventional sulfite pulping to maintain the calcium ions in solution it does cause degradation of the pulp. In addition to the improvement in strength obtained in the first stage of the present process the strength is further increased by the employment as a second stage of an alkaline solution extraction. It is believed that the alkaline extraction may increase the strength of the pulp by the removal of the shorter fiber length materials such as hemi-cellulose, etc., leaving only the longer fibered materials which have greater tensile strengths.

In addition to the tearing strength improvement the present process otters certain advantages with respect to the equipment which may be used to carry out the process commercially. For example, the first stage cooking operation of the present process may be carried out in conventional acid sulfite system digesters since the first stage may be carried out at pressures of pounds per square inch or less. Since there is no free $0 present in the first stage, the. pressure in the digester is only slightly higher than the saturated steam pressure generated by the cooking liquor at the cooking temperatureand substantially less than processes in which free S0 is present.

Current conventional acid sulfite method digesters are designed for maximum pressures of this order. The acid resistant linings of conventional digesters are of course suitable for the acid cooking liquor of the first stage of the present invention.

The absence of free S0 from the cooking liquor in addition to lowering the pressure requirements on the digester also simplifies the preparation of the: cooking liquor over that of conventional sulfite cooking liquor. Conventional acid sulfite cooking liquor must be prepared in pressurized equipment and stored in equipment capable of containing the pressure of the free S0 present. Since the present cooking liquor contains no free S0 it may be prepared and stored prior to use in. unpressurized equipment.

While the second stage of the present process is carried out with an alkaline liquor the equipment problems in connection with the second stage are simplified by the wide range of time, temperature and pressure conditions which may be utilized in this stage. The process of the second stage may be carried out at relatively high temperatures and pressures for a few minutes or completely equivalent results may be obtained by carrying out the process of the second stage at relatively low pressures and temperatures for longer periods of time. The second stage is particularly adaptable to being carried out in a continuous reactor, because of the short time required when the reaction is carried out at high temperatures and pressure. The high temperature and pres sure are not objectionable in continuous equipment because of the relatively small volume of the vessel required for suitable through puts of the materials.

The cooking liquor employed in the first stage of the present invention is a sodium bisulfite liquor having a pH between about 4 and 5. By employing the sodium bisulfite liquor having a pH of at least 4 the presence of free sulfur dioxide and consequently sulfurous acid is avoided. When a sodium bisulfite cooking liquor having a pH substantially greater than 5 is employed the effectiveness of the cooking liquor is so decreased that impractically long times are required for the digestion of the wood. The cooking liquor employed in the first stage of the present invention may be prepared by dissolving sodium bisulfite in water or by passing sulfur dioxide gas into an aqueous alkali solution such as sodium hydroxide until the desired pH level is obtained.

In the first stage of the present process wood chips of either hardwood or softwood or other conventional lignocellulosic raw materials such as bagasse, flex, hemp, etc., are cooked in the above described liquor under a pressure of about 145165 C., in a closed reaction vessel under the pressure autogenically produced for a period of about 4 to 14 hours. The liquor to wood ratio may vary from about 7.5 to 1 to 4 to l and is preferably about 6 to 1. The time and temperature-pressure are inter-dependent and equivalent results are obtained whether the cook is made for a short time at a high temperature-pressure or for a longer time at a lower temperature-pressure and there seems to be little difference in the strengths of the pulps produced at varying times and temperatures. The first stage is cooked to a yield of about 50-60 percent. By the term yield is meant the percentage of the dry wood charged to the digester which is converted into pulp during the cooking operation. Where the pulp is to be bleached after the pulping operation, cooking to a first stage yield of about 5055 percent with a maximum temperature of 155 C., or less is preferred. It will be noted that the cooking pressure at the temperatures of 155 C., or below are less than 95 pounds per square inch. The process under preferred conditions may thus be carried out in conventional sulfite digesters since these usually have a maximum pressure rating of about 95 pounds per square inch. If the process is carried out at the higher temperature the pulps obtained are bleachable but require more stringent bleaching conditions.

Upon completion of the first stage pulping operation the pulp may be blown from the digester and screened and washed. If the pulp is cooked to a higher yield for example of the order of 5560 percent yield it may be desirable to refine the pulp with conventional refining apparatus before carrying out the second stage of the present process.

The second stage of the present process comprises an alkaline extraction of the pulp obtained in the first stage. The alkaline extraction is carried out with an aqueous solution of an alkali such as sodium sulfide, sodium hydroxide, or sodium carbonate. The alkaline liquor employed in the second stage should contain between about and grams of alkali expressed as sodium oxide per liter of cooking liquor. The method of expressing conventional alkalis as sodium oxide is described in Pulp and Paper Manufacture, volume I, McGraw-Hill, 1950, page 380. The extraction stage is carried out in a pressurized vessel with the pulp obtained in the first stage suspended in the alkali solution. Weight or consistency of the pulp in the alkali solution is normally about 12 to 16 percent of the weight of the suspension. As in the first stage the pulp in the second stage may be treated for a brief period at high temperatures or for a longer period at lower temperatures. For example, equivalent reactions are obtained when the pulp is reacted in the second stage at 130 C., under a pressure of 25 pounds per square inch gauge for two hours and when the pulp is reacted at 195 C., under a pressure of 190 pounds per square inch gauge for a period of about 5 minutes. The pulp is usually reacted in the second stage for a period of time sufficient to give a yield of about -80 percent on the second stage. The resultant pulp normally has a permanganate number of approximately 15-18. Because of the wide range of temperature-pressure and time conditions under which the second stage may be carried out it will be apparent that considerable choice as to apparatus may be exercised. For example, if the second stage is carried out at low temperatures and pressures for moderate times for example C., for 2 hours at C., for /2 hour a batch operation in suitable equipment would be preferable. The second stage however might be carried out at high temperature-pressure in continuous reaction equipment where a 5' to 15 minute time in the reaction zone at temperatures of 195 degrees centigrade and pressures of 190 pounds per square inch might suitably be employed.

Upon completion of the second stage extraction the pulp may be removed from the reactor and washed in the conventional manner. For example, if a bleached pulp is desired the pulp may be treated with a conventional three stage chlorine dioxide bleaching process to attain a brightness of the order of 86-88 as measured with the General Electric brightness meter.

Now that the invention has been generally described it may be further illustrated by the following specific example.

EXAMPLE Five thousand parts by weight of spruce wood chips of a size conventionally employed in acid sulfite pulping operations were placed in the digester. To this was added a sodium bisulfite solution or liquor containing 20 percent by weight of sodium bisulfite and having a pH of 4. The liquor to wood ratio was 6 to 1. The digester was closed and the cook carried out in accordance with the following schedule: /2 hour to 120 C., 1 hour at 120 C., /2 hour to (3., and 5 hours at 155 C. The digester was then relieved and the pulp blown from the digester. An analysis of the pulp at this stage showed that there was a 53 percent yield and that the pulp had a permanganate number of approximately 31. The pulp was then refined for a brief time in an auxiliary unit, washed and screened. A 15 percent consistency suspension of the pulp was treated with 17 percent by weight of sodium hydroxide on the basis of the oven dry pulp. The suspension was placed in a pressure vessel and digested at 25 pounds per square inch at 130 C., for two hours. Upon completion of the digestion period the suspension was removed from the reaction vessel and the pulp separated from the solution and washed. A portion of the pulp was then bleached with a conventional three stage chlorine dioxide bleaching process to a General Electric brightness number of 88. The pulp was beaten to various freeness levels in at Valley beater and hand sheets then prepared in accordance with TAPPI (Technical Association of the Pulp and Paper Industry) Standard T205. The hand sheet prepared from pulp beaten for 33 minutes and having a freeness of 450 had a burst factor of 46.5 and a tear factor of 163. The burst factor was determined in accordance with TAPPI Standard T403 m-53, and the aceatsa tear factor in accordance with TAPPI Standard T414 m-49.

A comparison between pulps prepared in accordance with the process of the present invention, pulps prepared by a conventional sulfite process and pulps prepared by a two stage process similar to that of the present invention except that the first stage cooking liquor contained free S0 and had a pH of 1.5, is shown in the following table. Pulps 1 and 2 were prepared by the conventional acid sulfite process employing a calcium bisulfite liquor containing free S0 Pulps 4, 5, 6 and 7 were prepared as described in the example and diifer from each other only to the extent that the first stage pulping operation was carried out to the indicated yield percentage. Pulps 8, 9, 10 and 11 were prepared in the same manner as pulps 4, 5, 6 and 7 except that the first stage cooking liquor was an acid sodium bisulfite liquor having a pH of 1.5 and thus contained a substantial amount of free sulfur dioxide. The extent of the first stage of the pulping operation on the latter four pulps also differed from each other as indicated by the yield percentage. All of the pulps were subjected to a three stage chlorine dioxide bleaching process and were then beaten to a common burst factor. Hand sheets were prepared and the burst factors and tear factors were determined as previously described.

Table First Stage Burst Tear Pulp No. Yield, Factor Factor Percent free S0 and having a pH of about 4-5 and a concentra tion of about 20% sodium bisulfite by weight, at a liquor to wood ratio of between about 7.5 to 1 and 4 to 1, at a temperature of about 145 to 165 C., and at a pressure autogenically produced by said temperature, for a time of about 14 hours to 4 hours, to a pulp yield of about -60 percent by weight of the dry wood chips, separating the resultant pulp from said liquor and extracting said pulp with an aqueous alkaline liquor having an alkali content of 20-30 grams of alkali expressed as sodium oxide per liter, at a temperature of about 195 C. and a pressure of between 25 and 190 pounds per square inch gauge, for about 2 hours to 5 hours to a yield for the extraction step of about 75-80 percent by weight of the pulp obtained from the bisulfite cook.

2. The process of claim 1 wherein the alkali is sodium hydroxide.-

3. The process of claim 1 wherein the alkali is sodium sulfide.

4. The process of claim 1 wherein the alkali is sodium carbonate.

5. A two stage process of manufacturing an easily bleachable bisulfite pulp of improved tear strength in which the first stage consists of cooking wood chips in an aqueous sodium bisulfite liquor containing no free S0 and having a pH of about 4-5 and a cencentration of about 20% sodium bisulfite by weight, at a liquor to wood ratio of about dm 1, at a temperature of about ISO-155 C. and autogenically produced pressure, to a. pulp yield for the first stage of about 50-55 percent by weight of the dry wood chips, separating the resultant pulp from said liquor, and for the second stage extracting said pulp at a consistency of between 12-15 percent with an aqueous alkali liquor having an alkali content of 20-30 grams of alkali expressed as sodium oxide per liter of liquor at a temperature of -170 C. and autogenically produced pressure, for a time sufficient to provide a yield for said second stage of about 75-80 percent by weight of the pulp obtained from the bisulfite stage.

References Cited in the file of this patent UNITED STATES PATENTS 1,864,620 Richter June 28, 1932 1,899,637 Richter Feb. 28, 1933 1,940,136 Haglund Dec. 19, 1933 1,954,226 Richter Apr. 10, 1934 2,041,745 Dreyfus May 26, 1936 2,118,074 Dreyfus May 24, 1938 2,528,350 Farber Oct. 31, 1950 2,851,355 Battenberg Sept. 9, 1958 2,906,659 Dorland Sept. 29, 1959 

1. A METHOD OF MANUFACTURING BISULFITE PULP OF IMPROVED TEAR STRENGTH WHICH CONSISTS OF COOKING WOOD CHIPS IN AN AQUEOUS SODIUM BISULFITE LIQUOR CONTAINING NO FREE SO2 AND HAVING A PH OF ABOUT 4-5 AND A CONCENTRATION OF ABOUT 20% SODIUM BISULFITE BY WEIGHT, AT A LIQUOR TO WOOD RATIO OF BETWEEN ABOUT 7.5 TO 1 AND 4 TO 1, AT A TEMPERATURE OF ABOUT 145 TO 165*C., AND AT A PRESSURE AUTOGENICALLY PRODUCED BY SAID TEMPERATURE, FOR A TIME OF ABOUT 14 HOURS TO 4 HOURS, TO A PULP YIELD OF ABOUT 50-60 PERCENT BY WEIGHT OF THE DRY WOOD CHIPS, SEPARATING THE RESULTANT PULP FROM SAID LIQUOR AND EXTRACTING SAID PULP WITH AN AQUEOUS ALKALINE LIQUOR HAVING AN ALKALI CONTENT OF 20-30 GRAMS OF ALKLI EXPRESSED AS SODIUM OXIDE PER LITER, AT A TEMPERATURE OF ABOUT 120195*C. AND A PRESSURE OF BETWEEN 25 AND 190 POUNDS PER SQUARE INCH GAUGE, FOR ABOUT 2 HOURS TO 5 HOURS TO A YIELD FOR THE EXTRACTION STEP OF ABOUT 75-80 PERCENT BY WEIGHT OF THE PULP OBTAINED FROM THE BISULFITE COOK. 